For many years, antibiotic susceptibility testing (“AST”) has been used as a means for identifying part-particular groups or species of micro-organisms, or for identifying an antibiotic type or dose level most appropriate for dealing with a clinical infection. An internationally recognised standard procedure has been evolved using carrier devices, usually in the form of membrane disks, impregnated with known amounts of specific antibiotics. These disks are used in conjunction with Petri-dish plates containing a layer of growth medium, such as agar gel, to which material containing micro-organisms is applied. An arrangement of individual disks containing different amounts or types of antibiotics are placed on the gel surface at spaced intervals. Usually 6 or 8 disk are placed a circular array on the Petri-dish. The disks are normally of paper or other porous sheet material. The antibiotic diffuses out of each disk into the surrounding growth medium and establishes a radial concentration gradient around the disk. The relative susceptibility of the micro-organisms to the antibiotic is revealed by the diameter of the zone surrounding the disk within which growth of the micro organisms is inhibited. The detailed morphology of the zone can be indicative of the species or genus of micro-organism present. Visual interpretation of the results therefore requires considerable experience and skill, especially if adjacent zones of inhibition are large enough to overlap.
AST is applied on a very large scale worldwide, especially in clinical laboratories. In order to speed up and simplify the reading of such tests, various proposals have been made including the evaluation of the results using image analysis equipment, for example comprising a video camera linked to electronic information processing means such as a microprocessor. Reading systems of this type are available commercially. However, even with the equipment available today, there is still need for considerable skilled human input.
Because each culture plate will normally carry quite a number of disks each of which may be impregnated with a different amount of antibiotic, or indeed possibly an antibiotic different from that on other disks on the plate, it is absolutely essential that the identity of each disk is precisely determined. In accordance with a WHO-recommended standard, each disk bears a printed code which identifies the specific antibiotic on the disk and also its concentration. This code comprises a combination of letters and numerals which can be read easily by the human eye. The presently available “automated” imaging systems require that the operator reads the code on each disk by eye, and enters this information manually into the electronic processor, for example by means of a keyboard. Although optical character readers exist which can convert printed letters and numerals into electronic information, the printed characters must be aligned properly with the reading equipment if the optical character reader is to recognise the individual characters correctly. The disks used in AST are usually applied to the plates by means of mechanical applicators which release individual disks from an array of magazines, each holding a stack of identical disks. No attempt is made to control the orientation of the codes on the disks that drop from the applicator onto the plate surface. The disks are normally found on the plate with their printed codes orientated in a thoroughly random manner.